Re: Topband: EZNEC prediction help

[Terry Conboy <n6ry@arrl.net>]

At 12:52 PM 2005-02-08, Rick Darwicki N6PE wrote:
> A full size 1/4 wave 160 vertical shows a 1.41 db max 3D gain and an 
> average gain of -3.63. Used Mininec/medium ground.
>
> All of the bent, twisted and short stuff that will fit on my lot calcs at 
> a higher 3D max gain and less negative Average gain.  Would the gain 
> difference be due to the radiation pattern? IE might one of my bent 
> bananas be 3 db better than a full size antenna in some direction?  I'd 
> take the full size any day.
>
> One very interesting calc was adding a top wire to the 40/80 Create ground 
> plane with radials on the roof and fence. Then rerunning with as much of 
> the radials as possible laying on the ground (.1 ft). The Z shot way up.

Rick,

As Guy mentioned, use of MININEC ground is to be approached with caution, 
especially where you have horizontal wires much less than 1/2 wavelength 
high.  EZNEC, and the original MININEC, give overly optimistic gain since 
the antenna impedances are computed as if the ground were a perfect 
conductor but the far field patterns use your specified ground conductivity 
and dielectric constant.

As a result, "pure" verticals connected to MININEC ground give reasonable 
pattern shapes, but without real world ground losses (and their associated 
lower gain) unless you add suitable resistive "loads" in your model at the 
feedpoint.  When you mix in horizontal wires (as in a "T" or an "inverted 
L"), the results can really diverge from reality, particularly for short 
verticals.

Use of "High Accuracy" ground is better in many respects, but in the 
consumer grade version of EZNEC, does not allow buried radials or direct 
connection to ground.  Using radial wires just above the ground seems to 
give somewhat optimistic gain figures.  This can be mitigated to some 
extent by using lots of radials (30+) in the model.  If the real world has 
fewer radials, again you can tweak the model by adding resistive "loads" to 
simulate the added losses caused by the sparse radial field.

As W8JI has pointed out, the accuracy of NEC-based models near earth (even 
using "high accuracy" ground type) is questionable, both due to model 
inadequacies and to non-homogeneous and/or poorly characterized ground 
parameters.  It would seem that you can still use EZNEC to evaluate modest 
changes to an antenna design, but making direct comparisons of radically 
different configurations may be pushing it.

Terry N6RY

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